Corrosion and wear-resistant chill casting

ABSTRACT

A corrosion and wear-resistant chill cast part is formed from an iron composition comprising from 26 to 36 percent Cr; 0 to 10 percent Ni; 2 to 6 percent Mo; 0 to 3 percent Cu; 0 to 0.2 percent N; 0 to 1.5 percent Si; 0 to 1.5 percent Mn; 4 to 9 percent V; and 1.4 to 1.9 percent C. All percents are by weight of the total composition. The remainder of the composition is Fe and impurities.

BACKGROUND OF THE INVENTION

It is known in the art to use carbon-containing chromium chill castparts based on iron for stress from hydroabrasive wear. A cast of thattype is distinguished by having a carbon content that is greater than2.0 percent by weight. However, due to the high consumption of theavailable chromium to form carbide, those materials are not any moreresistant to corrosion than most non-alloy cast iron.

Typically, corrosion resistance is increased by reducing the carboncontent and increasing the chromium content of the composition, althoughreduced wear resistance must be accepted. A typical example of one ofthese materials is G-X 170 CrMo 25 2. One major disadvantage of thesematerials is that corrosion resistance in chemically aggressive media,such as, for example, acidic (pH 3) chloride-containing (50 g/l Cl)water from exhaust gas desulfuration equipment cannot be achieved,unless the chromium content is very high. However, high chromium levelsin iron-based alloys, such as the known materials G-X 160 CrNiMoCu 42 22 2 or G-X 140 CrMnNiMoCu 41 4 2 2 1, have the disadvantage of havingpoor mechanical properties and impaired casting properties.

For this reason, corrosion-resistant specialty steels whose wearresistance can be easily improved by lowering the carbon content (<0.5%)and the resulting smaller proportion, by volume, of carbides are usedfor the aforementioned aggressive media. The formation of chromiumcarbides reduces the chromium content of the basic structure, and thecorrosion resistance declines accordingly. Therefore, it is notadvisable to increase the carbon content further.

One way to avoid chromium depletion in a composition matrix with highercarbon contents is to add other carbide-forming elements. This isachieved in steels with low chromium contents (<20%), which are exposedto slightly corrosive media. An example is described in DE-A-42 02 339.The addition of niobium was considered to be particularly advantageous,because that alloy element forms pure MC carbide. The element vanadiumwas not considered beneficial because it reacts with chromium and ironto form composite carbides, which are less wear resistant.

Attempts to increase the chemical-tribologic resistance of thehigh-chromium material 1.4464 by adding small amounts of niobium,vanadium, or titanium are also known (M. Pohl, A. Ibach, A. Oldewurtel:New Cast and Forged Steel with Improved Chemical/Tribologic Resistance.Proceedings of the Fifth TRIBOLOGY Presentation 1991, Koblenz, pp.368-376). However, in some cases as a result of the low carbon content,corrosion resistance could be improved only slightly.

SUMMARY OF THE INVENTION

An object of the invention is to provide a metallic casting material(composition) that is distinguished by exhibiting high corrosionresistance in aggressive media and that approaches the wear resistanceof parts formed by standard chill casting processes.

This object is achieved by chill casting parts having the compositioncomprising from 26 to 36 percent Cr; 0 to 10 percent Ni; 2 to 6 percentMo; 0 to 3 percent Cu; 0 to 0.2 percent N; 0 to 1.5 percent Si; 0 to 1.5percent Mn; 4 to 9 percent V; and 1.4 to 1.9 percent C; all percents byweight of the total composition; wherein the remainder of thecomposition is Fe and impurities.

In addition to high resistance to corrosion and wear, this castingmaterial also has good casting properties. Thus, it can be produced inconventional special steel foundries. In addition, this chill castingmaterial has good working properties.

The object of the invention is achieved with a composition comprising achromium content from 26 through 36 percent by weight and a carboncontent of 1.4 through 1.9 percent by weight, which causes asufficiently high proportion by volume of carbides, and a vanadiumcontent of greater than 4 percent by weight, which reduces chromiumdepletion in the matrix by forming high-vanadium carbides. This makes itpossible to avoid the disproportionate increase in chromium content thatwould be necessary without the use of vanadium.

There are further advantages which result from adding vanadium. Vanadiumis an element from the fifth transition group, and its associatedcarbides are distinguished by possessing good wetting properties andlower solubility than chromium carbide in iron-based alloys. At the sametime, it is more soluble, in the liquid state, than niobium carbide is,which means that high-vanadium carbides typically do not form until alater stage of solidification or until the solid state occurs, producingan even spatial distribution of the carbides without gravitationalsegregation. This is a prerequisite for achieving good wear resistance.

In addition, high-vanadium carbides have been found to be equal to otherspecial carbides in imparting wear resistance. Moreover, thehigh-vanadium composite carbides are beneficial from the viewpoint offracture mechanics due to their form and the resulting low notch effect.The vanadium remaining in the matrix is not detrimental to themechanical properties.

The molybdenum content within the indicated limits is important forcorrosion resistance, particularly in acid media containing chlorideions.

The copper content is preferably less than 3 percent by weight in orderto reduce the risk of cracking when casting thick-walled parts. Lowcopper contents improve corrosion resistance in oxidizing media and aretherefore used in standard high-alloy compound steels. Another advantageof the copper content allowed by the material of the invention is thecapability of using recycled material from standard high-alloy caststeel for steel production.

By adding nickel, which forms austenite, in a concentration from 6 to 10percent by weight, the relationship of the ferrite and austenite phaseportions in the matrix can be adjusted according to definition. Thepositive properties of a compound structure in stainless steels areknown. The extremely high brittleness of chill casting varieties withhigh carbon contents and a carbide network in a ferrite matrix isavoided by the preponderantly interstitial high-vanadium carbides in theaustenitic phase. Because, in contrast to the ferrite phase, thosecarbides are not embrittled by precipitation of intermetallic phases orby segregation processes, the risk of cracks in the case of tensionbetween carbides and the matrix is not as great as in a purely ferriticmatrix.

To achieve a structure made up of a ferrite-austenite matrix withinterstitial carbides, heat treatment at the usual solution heattemperatures is necessary, which simultaneously improves workingproperties.

Additional heat treatment according to thetime-temperature-transformation curves of high-alloy steels also makesit possible to increase hardness by taking advantage of the knowntendency of ferrite to form precipitates, to further increase wearresistance.

A maximum of 4 percent by weight niobium can be added to the chillcasting composition, in order to allow the possibility of a secondaryprecipitation of eutectoid niobium carbides, which can increase wearresistance. The niobium content is limited to a maximum of 4 percent byweight to avoid the precipitation of primary niobium carbides in themolten mass, because they strongly segregate due to the differencebetween their density and that of the matrix.

In contrast with the known chromium chill casting parts made from knownprocesses, the material of the present invention has very lowsusceptibility to corrosion, particularly to selective corrosion, due tothe low chromium content of the carbides.

An additional advantage of this material is that at a given wearresistance, the corrosion resistance can be adjusted by varying thealloy elements that are relevant to corrosion chemistry, according tothe profile of the given requirements. However, it must be kept in mindthat as the alloy content increases, production (casting and metalcutting properties) becomes more difficult.

In regard to the combination of corrosion resistance and wearresistance, the materials of the present invention are superior to thechill casting materials of the prior art that have been used forhydroabrasive wear.

This can be shown using the example of a comparison of three materialcompositions of the invention that are contrasted with four chillcasting process materials of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic illustration of the wear rates of materials underhydroabrasive wear; and

FIG. 2 is a graphic illustration of the corrosion rates in a highly acidmedium (pH 0.5; 10 g/l Cl--; 60° C.).

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

To determine the wear rates shown in FIG. 1, a model wear device wasused in which the corrosion agent was a 1:1 mixture of water and quartzsand having a grain size of 0.9-1.2 mm. Each test lasted two hours. Arotation speed of 3,000 l/minute was used. Each sample part was 55 mm indiameter and 5 mm thick.

The Y-axes of the diagrams shown in FIGS. 1 and 2 indicate the wear inmm/a. On the X-axes, the letters A through D indicate the materials ofprior art, which are explained in Table 1 below, while references E (1)through E (3) are the three compositions of the invention, thesecompositions are shown in Table 2.

                  TABLE 1                                                         ______________________________________                                        Materials of prior art used in the tests                                      Reference        Short name                                                   ______________________________________                                        A                G-X 250 CrMo 15 3                                            B                G-X 170 CrMo 25 2                                            C                G-X 3 CrNiMoCu 24 6                                          D                G-X 40 CrNiMo 27 5                                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Alloy composition of the materials in accordance with                         the invention that were used in the tests                                     Ref. C      Si    Mn   Cr   Ni   Mo  Cu   V    Fe                             ______________________________________                                        E(1) 1.5    0.8   0.6  26.6 7.9  2.6 1.8  5.2  Remainder                      E(2) 1.5    1.2   0.8  30.1 8.2  2.4 1.7  5.0  Remainder                      E(3) 1.8    0.8   0.9  31.8 8.7  2.8 1.8  8.9  Remainder                      ______________________________________                                    

We claim:
 1. A corrosion and wear-resistant chill cast part wherein saidpart is formed from an iron composition comprising from 26 to 36 percentCr; 0 to 10 percent Ni; 2 to 6 percent Mo; Cu being present in an amountless than or equal to 3 percent; N being present in an amount less thanor equal to 0.2 percent; 0 to 1.5 percent Si; 0 to 1.5 percent Mn; 4 to9 percent V; and 1.4 to 1.9 percent C; all percents by weight of thetotal composition; wherein the remainder of the composition is Fe andimpurities.
 2. The chill casting part according to claim 1, wherein thenickel content is from 6 to 10 percent by weight.
 3. The chill cast partaccording to claim 1, wherein said composition comprises up to 4 percentby weight of niobium.
 4. The chill cast part according to claim 2wherein said composition comprises from up to 4 percent by weight ofniobium.
 5. A chill cast part that contacts a flowing corrosive media,optionally containing solids; wherein said part is formed with an ironcomposition comprising from 26 to 36 percent Cr; 0 to 10 percent Ni; 2to 6 percent Mo; Cu being present in an amount of less than or equal to3 percent; N being present in an amount less than or equal to 0.2percent; 0 to 1.5 percent Si; 0 to 1.5 percent Mn; 4 to 9 percent V and1.4 to 1.9 percent C; all percents by weight of the total composition,wherein the remainder of the composition is Fe and impurities.
 6. Achill cast part according to claim 5, wherein the composition comprisesfrom 0 to 4 percent by weight Niobium.
 7. A chill cast part according toclaim 5, wherein said part is a pump.
 8. A chill cast part according toclaim 5, wherein said part is an armature.